Impact of wetting–drying cycles on the microstructure and mechanical properties of lime-stabilized gypseous soils

• Durability of lime-treated gypseous soil is problematic against wetting–drying events.
• Calcium hydrates (CSH and CAH) and ettringite are formed in lime-stabilized soils.
• P-wave velocity can be used in the field to assess the behavior of gypseous soil.
• Lime added must be correlated to the mineralogy and the gypsum content of the soil.

Gypseous soil is a problematic soil due to its high water sensitivity. Its behavior depends on its gypsum content and it loses much of its strength when saturated. Lime stabilization is a traditional technique used to stabilize soils in civil engineering works, mainly road construction. In order to assess the long-term stability of lime stabilized gypseous soils, the effect of wetting–drying cycles on the mechanical behavior of fine-grained soil with different gypsum contents (0, 5, 15 and 25%) was investigated. The soil samples were stabilized with 3% lime and subjected to different curing times of up to 28 days, then exposed to 6 wetting–drying cycles of 96 h each. Results showed that the strength of the compacted soil samples increased with curing time and that the maximum unconfined compressive strength was reached for the soil samples with 5% gypsum. While the wetting–drying cycles had a detrimental effect on the compressive strength and P-wave velocity of the soil samples, these effects varied with gypsum content. Greater loss in strength occurred in soil samples with 25% gypsum. These changes can be explained by the increase in water content during these cycles, crack propagation during drying and gypsum dissolution during wetting. Mineralogical and microscopic studies showed the formation of Ca-hydrates composed of different proportions of Ca, Si, and Al, responsible for the increased strength of stabilized soil samples. Ettringite was also found in stabilized soils and acts in the opposite direction to reduce the strength of soil samples. The durability of the tested gypseous soils treated with 3% of lime against wetting–drying cycles is not guaranteed. To ensure the durability of these problematic soils against environmental conditions, other percentages of lime and another stabilizer such as cement need to be tested.